Fermentative production of tetracycline



United States Patent 1 Claim ABSTRACT OF THE DISCLOSURE A process is disclosed for the microbiological production of tetracycline. The process comprises cultivating a Streptomyces lusitanus microorganism in an aqueous nutrient medium containing assimilable sources of carbon and nitrogen and chlorine ion-yielding salt-and recovering tetracycline values from the fermentation broth. In the process the production of chlorotetracycline is substantially entirely suppressed and the produciton of tetracycline is enhanced.

This application is a continuation of application Ser. No. 425,612, filed Jan. 14, 1965, and now abandoned.

The present invention relates to a fermentation process for producing tetracycline and for the recovery of the latter from the fermentation medium.

The microbiological production, fermentatively, with the aid of Streptomyces lusitanus, of chlortetracycline in a medium containing chlorine ions, and of tetracycline in a medium free from chlorine ions, has heretofore been disclosed.

The present invention is concerned with the microbiological production of tetracycline, in the presence of chlorine ions in the fermentation medium and without simultaneous chlortetracycline coproduction, with the aid of a Streptomyces natural isolate or of a mutant thereof, and more especially with the aid of a preferred, so-called industrial, strain. In view of the similarity thereof to Streptomyces lusitanus with respect to taxonomical characteristics, the new species is called Streptomyces lusitanus var. tetracyclini. The preferred and industrially highly useful strain, Streptomyces lusitanus var. tetracyclini 106-T, has been carefully selected under ultraviolet light and a culture thereof has been deposited in the National Collection of Industrial Bacteria, Aberdeen, Scotland, with accession No. NCIB 9500.

Various microorganisms known to produce tetracycline comprise Streptomyces aureofaciens, Streptomyces sayamaensz's, Streptomyces viridfaciens, Streptomyces psammoticus, Streptomyces persimilis, etc.; however, Streptomyces lusimnus var. tetracyclini is distinguished by producing tetracycline with such a high yield that it surpasses by far the production of any other known species.

The fact that Streptomyces lusitanus var. tetracyclini 106-T produces tetracycline in presence of chlorine is of great industrial interest, as tetracycline is actually the most important of all the teracyclines from the point of view of production and sales, and the preparation of chlorine ion containing culture media is cheaper than that of the dechlorinated media.

Streptomyces lusitanus var. tetracyclini was isolated from the soil in Brive-la-Gaillarde, France, in a place denominated Migoul. In accordance with the criteria of the classification systems of Ettlinger et al., and Pridham et al. it has been determined to be a species independent from those cited above.

Streptomyces lustinanus var. tetracyclini 106T produces, according to the conditions of fermentation, 9 to 12 grams of tetracycline per liter (g./l.) of fermented broth. It should be noted that in the present specification the designation g./l. always refers to tetracycline hydrochloride in accordance with international custom.

Strepzomyces lusitanus var. letracyclini 106-T has straight to flexous, sympodially branched sporophores, forming no spirals and no verticals but only open hooks, the surface of spores being smooth when observed through an electron microscope. The form of spores is ovoid, measuring 0.60.8 by 1.3-1.7 the color en masse of the spores goes from light-gray to olive buff; generally it sporulate very well, like Streptomyces lusitanus (CBS 10lA), on certain rich media.

According to Ettlingers classification, Streptomyces lusitanus var. tetracyclini 106T belongs to the group: straight to flexo-us, sympodially branched, griseus to cinnamoneus, smooth spores, with melanin pigment.

According to Pridham et al.s classification system, Streptomyces lusitanus var. tetracyclini 106-T belongs to the group: Retinaculum Apertum, series gray to olive buff.

With a view to comparing and difierentiating Streptomyces lusitanus var. tetracyclini 106-T, there are hereinafter set forth the cultural characteristics of Streptomyces lusitanus, (CBS 10l-A),

Streptomyces lusitanus var. tetracyclini 106-T, NCIB 9500,

Streptomyces aureofaciens, NRRL 2209,

Streptomyces viridifaciens, ATCC 11989 on 14 various media after 16 days of incubation at 26 C.

(1) Corn steep liquor medium 0.6% of the composition:

Agar agar grams 10 Corn steep liquor, 50% do 3 Glucose do 15 (NH HPO do 2.5 KH NO do 7.5 do- 1 MnCl do 0.002 CuSO .5H O do 0.002 ZnSO .7H O do 0.025 Water cc 500 pH 7, after sterilization.

The aerial mycelium of Streptomyces lusitanus is lightgray, of Streptomyces lusitanus var. tetracyclini 106-T dark-brown, of Streptomyces aureofaciens dark-gray, and of Streptomyces viridifaciens gray to almost black. Streptomyces lusitanus var. tetracyclini l06T produces darkbrown diifusible pigment compared to that of Streptomyces viridifaciens which is lighter. All strains grow Well.

(2) Corn steep liquor medium 0.4% having the same composition as medium (1), but the quantity of corn steep liquor being 2 grams instead of 3 grams. The four strains grow similarly to medium No. 1, although growth is slower.

(3) Gelatin medium of the composition:

Meat extract grams 1.5 Peptone do 2.5 Gelatin do Distilled water cc 500 pH 6.2, before sterilization.

The four cultures grow similarly, no liquefaction being present. Streptomyces aureofaciens and Streptomyces viridifaciens do not produce difiusible pigment, while Streptomyces lusitanus produces a yellow pigment and Streptomyces lusitanus var. tetracyclini 106-T produces a yellowish-brown pigment.

(4) Czapek-Dox-Dextrine medium of the composition:

Dextri-ne grams 5 NaNO do 1 K2HPO4 (.lO 0.5 dO KCl do 0.25 FeSO, A small crystal Agar agar grams 7.5 Distilled water cc 500 pH 6.8, after sterilization.

Streptomyces lusitanus var. tetracyclini 106-T.

(6) Bennett Agar of the composition:

Yeast extract grams 0.5 Meat extract do 0.5 Hydrolyzed casein do 1 Glucose zdo 5 Agar agar d 7.5 Distilled water cc 500 The pH, adjusted to 7, gives after sterilization pH The growth of the four cultures is similar to that obtained in media (1) and (2), but Streptomyces aureofaciens and Streptomyces viridifaciens do not form aerial mycelium, and the spores en masse of Streptomyces lusitanus var. tetracyclini 106-T are brown-colored, those of Streptomyces lusitanus being slightly grayish-white. Streptomwces aureofaciens and Streptomyces viridifaciens do not form difiusible pigment, while Streptomyces lusitanus and Streptomyces lusitanus var. tetracyclini 106-T produce a dark-brown pigment.

(7) Glycerin-asparagin medium of the composition:

Glycerin grams Asparagin do 0.25 Meat extract do 1 K HPO do 0.25 Agar agar do 7.5 Distilled water cc 500 pH 6.9, after sterilization.

NaNO gram 1 K HPO do 0.5 MgSO .7H O dO KCl do 0.25 FeSO, A small crystal Agar agar "grams" 7.5 Distilled water cc 500 pH 7.1, after sterilization.

Streptomyces lusitanus and Streptomyces lusitanus var. tetracyclini 106-T do not grow on this medium, as likewise in all media containing inorganic nitrates as sole source of nitrogen. Streptomyces aureojaciens and Streptomyces viridz'faciens grow well, without any aerial mycelium, presenting the typical light-yellow color.

(9) Emerson agar of the composition:

Yeast extract grams 2 Soluble starch do 7.5 K HPO do 0.5 .d0 Agar agar do 10 Distilled water ce 500 pH 7, after sterilization.

Streptomyces lusitanus grows well on this medium with brown-lilac color, without aerial mycelium, and brown difiusible pigment; Streptomyces lusitanus var. tetracyclini 106-T grows similarly to the Streptomyces lusitanus strain but forms a light olive buff aerial mycelium. Streptomyces aureofacz'ens is characterized by faint growth, without formation of pigment; Streptomyces viridifaciens grows better, forming scant aerial mycelium and light-yellow diffusible pigment.

(l0) Czapek-Dox-Starch medium of the composition:

Soluble starch 5 grams. NaNO lgram. K HPO 0.5 gram. MgSO .7H O 0.25 gram. KCl 0.25 gram. FeSO A small crystal. Agar agar 7.5 grams. Distilled water 500 cc.

pH 7, after sterilization.

The results are similar to those of the other nitratecontaining media. Streptomyces lusitanus and Streptomyces lusitalzus var. tetracyclini 106-T do not grow, while Stroptomyces aureofaciens and Strcptomyces viridifaciens grow with yellow-orange color, without pigment or aerial mycelium.

(ll) Litmus milk: pH 6.45 before sterilization.

All four cultures grow very slowly in a ring around the assay tube without digesting or coagulating the milk. There is a slight but apparent difference in the pH of Streptomyces lusitanus and that of Streptomyces lusilanus var. tetracyclini 106-T on one hand and on the other in that of Streptomyces aureofaciens and Streptomyces viridifaciens.

(l2) Nutrient medium of the composition:

Meat extract grams 1.5 Peptone do 2.5 Agar agar do 7.5 Distilled water cc 500 The pH after sterilization is 6.8.

The four cultures are similar but Streptomyces Iusitanus and Streptomyces lusitanus var. telracyclini 106-T form a dilfusible pigment darker than that of Streptomyces aureofaciens and Streptomyces viridifaciens.

(l3) Glucose-asparagin medium of the composition:

Glucose grams 1 Asparagin do 0.25 Meat extract do 1 K HPO do 0.25 Agar agar do 7.5 Distilled water cc 500 The pH after sterilization is 6.9.

Streptomyces lusitanus, Streptomyces aureofaciens and Streptomyces viridifaciens grow similarly with aerial mycelium, but Streptomyces lusitanus and Streptomyces viridifaciens form a dirty-yellow diflusible pigment. Streptomyces lusitanus var. tetracyclini 106-T does not form aerial mycelium; the difiusible pigment being similar to that of S treptomyces lusitanus.

(14) Sucrose-Dextrine-Nitrate medium of the composition:

pH 7, after sterilization.

Streptomyces lusitanus and Streptomyces lusitanus var. tetracyclini 106-T do not grow on this medium, while Streptomyces aureofaciens presents a light-yellow vegetative growth, the vegetative growth of Streptomyces viridifaciens being dark-orange.

The utilization of carbon source by Streptomyces lusitanus was not possible to determine in the beginning by applying Okhis method (OKHI, N.: Kitasoto Arch. Exp. Med. 25, 209, 1953) in view of the fact that this strain does not grow in presence of inorganic nitrogen as sole source of nitrogen. The method of Pridham and Gottlieb as described by Zahner and 'Ettlinger in Arch. f. Mikrobio1., vol. 26, 307, 1957, was then applied. The observations were made on the tenth day. The annexed table of comparison relates solely to the six carbon sources considered by Zahner and 'Ettlinger as being characteristic and significant for the determination of species.

Consequently from the point of view of carbon source and according to Ziihners and Ettlingers classification system, Streptomyces lusitanus var. tetracyclini 106-T and Streptomyces lusitanus 101-A belong to group 1112 and Streptomyces aureofaciens to group 11111.

Accordingly, the taxonomical differentiation from the other known tetracycline-producing species has been made by comparing the original isolates as well as the industrial strains.

Consequently, as the various selections of industrial strains do not grow or grow very scantily on Ettlingers medium of melanoid pigment formation (yeast extract 0.5 gram, L-tyrosine 0.5 gram, NaCl 4.25 grams, agar 8 grams, tap water 500 cc.), the evaluation of melanoid pigment formation was, therefore, not possible with Streptomyces lusifanus var. tetracyclini 106-T. The original isolates (mother strains of the industrial ones) of Strept0 myces lusitanus and Streptomyces lusitanus var. tetracyclini were specially studied on this medium. Streptomyces Iusitanus forms, within 2 days, on this tyrosine medium, a

reddish pigment (consisting, with all probability, of either hallochrome or 5,6-dihydroxyindole), which slowly oxidizes into a dark pigment the color of which is not altered with the pH variation. The ultraviolet absorption curve of the extracted pigment is identical to that described by Schmidli for melanin pigment in Helvetica Chimica Acta, 38, 1078, 1955. Streptomyces lusitanus var. tetracyclini forms immediately after 34 days (without forming the intermediate red pigment) a black pigment similar from every point of view to synthetic melanin obtained by oxidation of dopa 3-(3-4-dihy-droxyphenyl)-a-alanine).

Thus both Streptomyces lusitanus. and Streptomyces lusitanus var. tetracyclini form melanoid pigment on tyrosine media, although through two different pathways, whereas neither Streptomyces aureofaciens nor Streptomyces viridfaciens form melanoid pigment by any of those pathways.

Utilization of Carbon Source by- Streptomyces Streptomyces aureofaciens lusz'tamts according to Carbon source Benedict 2 Streptomyces Applicants Ztihner and (strains: viridifaciens 3 106-T 101-A assays (strain Ettlinger l NRRL-B N RRL 2209) 1286, 1287, 1288, 2209) Rhamnose. to Raflfinose. dy s to d-Fructose. l-Arabinosed-Mannitol 1 Zahner and Ettlinger, Arch. t. Mikrobiol., 26, 307, 1957. 2 Benedict et 8]., Appl. Microbiol. 3, 1, 1955.

3 U.S. Patent No. 2,886,595.

: No utilization.

+: Positive utilization.

..2 No data available.

(): Improbable utilization.

(+): Probable utilization.

+/: Positive or negative utilization according to strains.

The present invention is directed to an industrially useful process, and as stated Streptomyces lusitanus var. tetracyclini 106-T is an industrial strain selected from the original isolate. Cultures of the original soil isolates of Streptomyces lusitanus and Streptomyces lusitanus var. tetracyclini have been preserved and are on deposit in various culture collections.

The mother strain of Streptomyces lusitanus var. tetracyclini has a tendency to form sclerotia on certain media, as for instance on Ettlingers tyrosine-agar.

Streptomyces lusitanus 101-A and Streptomyces lusitanus var. tetracyclini 106-T as well as its mother strain grow at 50 C. No verticil formation was observed with either Streptomyces lusitanus or Streptomyces lusitanus var. tetracyclini, while in ripe cultures of both Streptomyces aureofaciens and Streptomyces viridifaciens verticils are often 'found.

The two mother strains differ morphologically from the industrial ones in that both belong to group Spira.

The following table summarizes the most important taxonomical dilferences existing between Streptomyces lusitanus and Streptomyces aureofaciens.

Streptomyces lusitanus Characteristics of Differentiation Var. tetracyclim' 106-1 Sporophore morphology Sympodial branching; Sympodial branching; no Monopodial branching; vertino verticils. verticils. cils present. Color of spores en masse Cinnamoneus to griseus..- Cinnamoneus to griseus Cinereus.

Formation of melanoid pigment on tyrosine medium. positive).

No growth (mother strain No growth (mother strain Negative.

positive).

Utilization of inorganic nitrogen source... Negative Negative Positive. Utilization of carbon source according to Belongs to group IIIe. Belongs to group IIIe Belongs to group IIIa.

Ziihner and Ettlinger.

The above comparison shows that Streptomyces lusitanus var. tetracyclini 106-T differs only silghtly (within the range of the scientifically admitted variation of a species) from the chlortetracycline producing Streptomyces lusitanus (CBS 101-A) and that the differences are very accentuated and significant between Streptomyces Iusitanus var. tetracyclini and Streptomyces aureofaciens or Streptomyces viridifaciens.

The simple fact that Streptomyces lusitanus var. tetracyclini 106-T produces 2 to grams/ liter of tetracycline more than the highest value found in the literature for a tetracycline-producing microorganism proves indirectly its independence and represents a considerable industrial improvement for the production of tetracycline.

The use of a hydrolysate of starch makes it possible to increase the amount of carbon source per liter of broth, such an increase being accompanied by a substantial increase of yield in antibiotic activity per liter of fermented broth, although both Streptomyces lusitanus and Streptomyces lusitanus var. tetracyclini hydrolyze starch.

The best results are obtained with a hydrolysate, containing not more than 20% of unhydrolyzed corn starch and not more than 20% of the diand monomers, the remaining 60% consisting of oligomers. When using such a semi-hydrolysate of starch, one can add, up to 65 grams/liter, to the medium without it becoming too viscous. In comparison, the addition of 65 grams/liter of corn starch or even of considerably lesser quantities will cause a near solidification of the media, thus making a submerged fermentation unpracticable. The yields in tetracyclin'e usually surpass 12 grams/liter under such conditions. On the other hand, this increase in carbon source makes it possible to eliminate the use of lard oil as carbon source, without a considerable loss in yield, which represents a great advantage in countries where lard oil has to be imported (as in Moslern countries). Foam control can be then achieved by using a small amount of silicon antifoam agents.

The aqueous nutritive medium for industrial fermentation by Streptomyces lusitanus var. tetracyclim' 106-T contains assimilable sources of organic nitrogen, carbon and mineral salts, including chlorides, favorable to the growth of the microorganism.

As source of nitrogen, use can be made of hydrolysate of casein, extract of malt, barley or corn, corn steep liquor, peanut-meal, soya meal, etc. Inorganic nitrates are not utilizable.

As carbon source, use can be made of various carbohydrates such as glucose, dextorse, maltose, trehalose, starch, hydrolyzate of starch, dextrine, animal or vegetable fats or fatty acids.

The quantity and proportions of nutrients are exemplified in the examples.

Addition of N,N-dibenzylethylenediamine (DBED) as acetate or lactate in a concentration of between 0.01 and 1.5 grams per liter provokes a higher yield compared to that of parallel simultaneous runs carried out without the addition of DBED. DBED is added to the broth, preferably divided into various portions, during the fermentation. The fermentation is carried out at a temperature comprised between 24 to 30 C. under stirring and strong aeration comprised between 0.1 to 4.0 parts/ minute of the volume to be fermented according to the phase of fermentation.

The time of fermentation required to obtain the highest yield varies from 96 to hours according to the fermentation conditions.

The fermented broth does not contain any measurable amount of chlortetracycline so selective is the tetracycline production.

An indirect proof that Streptomyces lusitanus var. tetracyclini is a species independent from Streptomcyces aureofaciens is the fact that the color imparted to the whole harvested mash (containing no chlortetracycline) is essentially different from that described for Streptomyces aureofaciens in US. Patent No. 3,092,556. The spectrophotometric reflectance of the whole harvested mash of Streptomcyces lustianus var. tetracyclini 106-T is measured in one centimeter glass cells at wavelengths between 400 to 700 my, using magnesium carbonate as reference. The reflectance values (R) and the corresponding wave lengths are both plotted linearly on a graph, establishing a characteristic reflectance curve. A straight line is then drawn through the linearly plotted reflectance curve intercepts at 400m, and SSOm and the vertical distance measured between this line and the reflectance curve at 420m and 430m The values are represented by the symbols AR and AR respectively. In the case of Streptomyces aureofaciens, according to US. Patent No. 3,092,556, AR is greater than AR and in the case of Streptomyces lusitanus var. tetracyclini 106-T AR is always smaller than AR although no demonstrable amount of chlortetracycline 50 mcg./ ml.) is present in the mash of Streptomyces lusitanus var. tetracyclini 106-T.

Once the fermentation is completed, the active principle is extracted and purified after the manner disclosed in the illustrative examples, infra.

The final product obtained in the form of hydrochloride of tetracycline has the following physical and chemical characteristics: decomposition around 214 C.; optical rotation 258 (c. 0.5 in 0.1 NCHl); it is very soluble in water, methanol and ethanol. Elementary analyses of the trihydrate base correspond to the formula C H N O -3H O, having a melting point of 175 C. (dec.).

The product obtained corresponds, both in the form 0 fbase or as hydrochloride, in all physical, chemical and biological characteristics, to that described in the literature as tetracycline.

The industrial advantages of the present process using the natural variety of Streptomyces lusitanus var. tetracyclini 106-T consist mainly in the very high yields obtained and the cheap culture medium required.

The following non-limitative examples serve to illustrate the present invention by way of presently preferred embodiments.

EXAMPLE 1 All media were prepared with tap 'water. 1 liter of a sterilized medium having the following composition:

Grams Corn steep liquor 50% 10 Sugar l0 CaCO 1 (NH HPO 2 KH PO 2 MgSO 7# O 0.25

Water, ad 1000 cc. pH 6.4 after sterilization.

was inoculated with 1 ml. (milliliter) of a suspension, in physiological saline of ripe spores of Streptomyces lusitanus var. tetracyclini 106-T and incubated at 26 C. in a flask on a rotative agitator during 36 hours.

Afterwards, a prefermenter having a working volume of 150 liters, containing a medium of the following composition:

Grams Cornsteep liquor 50% 35 CaCO 13 Sugar Water, ad 1000 cc. pH 6.7 after sterilization.

was inoculated with the above mentioned 36 hour culture and fermented at 26 C. under agitation and sterile aeration for 24 hours.

Finally, a fermenter having a useful capacity of 6000 liters, containing a medium of the following composition per liter tap water:

Grams Corn step liquor 50% 28 Calcium carbonate 14 Starch 38 (NHQ SQ, 5.7 NH Cl 1.5 MnSO .4H O 0.05 CoCl .6H O 0.002 ZnSO 0.05 Peanut meal 25 Lard oil 35 pH 6.7-6.8 after sterilization.

was inoculated with the above mentioned 24 hour culture and fermented at 30 C. during 24 hours with an aeration of 1.5 liters per minute per liter of 'broth. Then the temperature was lowered to 26 C. and the aeration slowly increased in such as way that, at the end of 140 hours, it attained 4 liters per minute per liter of broth.

After 140 hours of fermentation there are obtained 11.1 grams of tetracycline per liter, no chlortetracycline being detectable 50 mcg./ml.).

EXAMPLE 2 The procedure is as in Example 1 but there is added to the last culture medium 0.5 gram per liter of N,N- dibenzylethylenediamine diacetate in 4 equal portions at 0, 36, 72 and 98 hours of fermentation. After 140 hours, the quantity of tetracycline obtained is 12.3 grams per liter.

EXAMPLE 3 The fermented mash obtained in Example 1 is acidified to pH 1.5 with 25% sulphuric acid and filtered with a drum filter, the cake is then extracted twice with water at pH 1.5. To the combined filtrate there is added 18 kg. of Versene (ethylenediamine tetraacetate, sequestering agent) and 16.5 kg. of DBED diacetate, and afterwards the pH is slowly adjusted to 9.7 with 12% ammonia. After 3 hours of agitation, the precipitate which consists essentially of impure DBED tetracyline complex having the formula of DBED.Ca.(tetracycline) is filtered off. The wet precipitate is then suspended in water, acidified with a aqueous solution of oxalic acid to pH 1.5 under agitation. Then the solution is filtered and the pH is adjusted to 5.8 with a 10% aqueous sodium hydroxide solution. Tetracycline base precipitates. The precipitate is filtered, washed and dried under vacuum at 65 C. Effective yield 83.5% calculated in activity.

EXAMPLE 4 The procedure is as in Example 3 but instead of DBED, there is added 16 kg. of N,N-dibenzylethylene- 10 diimine (C H .CH=N.CH .CH .N:CH.C H at pH 6, effective yield in the form of base is 86% calculated in activity.

EXAMPLE 5 The procedure is as in Example 1, but the composition of the culture medium of the main fermenter is replaced by the following, per liter of tap water:

Grams Corn steep liquor 50% 31 Calcium carbonate 14 Semi-hydrolyzate of starch containing not more than 20% of unhydrolyzed corn starch and not more than 20% of monoand dimers, the re- The final yield is 12.2 grams per liter. No chlorotetracycline is detachable in the mash 50 mcg./cc.). From the whole harvest mash reflectance curve there are obtained from AR420=18 and AR43o=92-4.

EXAMPLE 6 The procedure is as in Example 5, but the lard oil and peanut oil are omitted and foam formation is controlled by adding silicon antifoam agent. Yield after 136 hours of fermentation: 10.1 grams per liter. No chlorotetracycline is detectable in the mash.

EXAMPLE 7 40 cc. sterilized culture medium of the formula described in Example 1 for final fermentation are inoculated with germinating spores of Streptomyces lusitanus var. tetracylini 106-T in a 300 cc. Erlenmeyer flask and incubated for 7 days at 28 C. in a rotary shaker. The reflectance curve of the whole harvest mash is determined and the AR values calculated, giving: AR =3.8 and AR43U=34.2.

What is claimed is:

1. In a process for the production of tetracycline values by the cultivation of a Streptomyces lusitansus microorganism in an aqueous nutrient medium containing assimilable sources of carbon and nitrogen and chlorine ionyielding salt, and recovering tetracycline values from the fermentation broth, the improvement according to which the microorganism is Streptomyces lusitanus var. tetracyclini 106-T (NCIB 9500) and starch hydrolyzate is used as carbon source, hydrolyzate being semi-hydrolyzate of starch containing not more than 20% of unhydrolyzed starch and not more than 20% of monomers and dimers, the remainder being oligomers, whereby the production of chlorotetracycline is substantially entirely suppressed and the production of tetracycline is enhanced.

References Cited UNITED STATES PATENTS 3,092,556 6/1963 Growich et al.

FOREIGN PATENTS 920,126 3 1963 Great Britain.

936,314 9/ 196 3 Great Britain.

LIONEL M. SHAPIRO, Primary Examiner. 

